Reland "Enable R8 full mode by default for apps"

This is attempt 4 at landing this change. Prior attempts were
reverted due to downstream test breakages not in presubmit.
Those issues have been resolved, and additional manual heavy
presubmit tests were run to ensure stability.

Observed APK savings: ~24MB

This reverts commit 74a5c2ec.

Reason for revert: Fixed ManagedProvisioningTests

Bug: 215530220
Change-Id: I223228a345fa1a3bad52fb3e91c744107e04fc07